The present invention relates to a light-converging optical system for at least one of recording and reproducing for an optical information recording medium, an optical pick-up device, a recording/reproducing apparatus, an aberration correction element and an objective lens.
Heretofore, an optical disk has been used widely for accumulation of music information and image information and for storage of digital data such as storage of computer data. In recent years, there has been a strong demand for a large capacity of the optical disk, with the incoming of information age, and there has been commercialized DVD (digital versatile disk) wherein recording capacity (recording density) per unit area is enhanced to seven times that of the disk in the same size as CD (compact disk).
In the optical disk, it is possible to realize improvement of recording density, by making a spot diameter obtained from an optical pickup optical system to be small. Since the smallest diameter of the spot is proportional to λ/NA (λ represents a wavelength of a light source, and NA represents the numerical aperture of the optical system, in this case), a short wavelength of a light source used in the optical pickup device and a high numerical aperture of an objective lens arranged to face an optical information recording medium in the optical system, are effective for making the spot diameter to be small.
Among the foregoing, with respect to the short wavelength of the light source, a violet semiconductor laser and an SHG violet laser which emit light with a wavelength of about 400 nm seem to be put to practical use in the near future, because researches of them have been advanced. If these short wavelength light sources are used, information of about 15 GB can be recorded for the optical disk having a diameter of 12 cm which is the same as that of DVD, even when using an objective lens having NA of 0.65 which is the same as that of DVD.
Further, with respect to the high numerical aperture of an objective lens, there have been advanced researches of an objective lens that is composed of one or two lenses and has a high numerical aperture of NA 0.85. By using a combination of the aforesaid short wavelength light source and an objective lens with high numerical aperture of NA 0.85, information of about 25 GB can be recorded for the optical disk having a diameter of 12 cm, which makes it possible to attain the higher density.
With the background like this, each company is buckling down to research and development for high density and advanced optical disks each employing a violet light source having a short wavelength and a high numerical aperture objective lens (hereinafter referred to as “high density DVD” in the present specification).
Incidentally, in the optical pickup device, plastic lenses are used as an objective lens in large quantities, because they are advantageous for mass production. However, it is known that the refractive index of the plastic lens changed by temperature changes is greater than that of a glass lens by magnifying power of a two-digit number.
If an ambient temperature for the objective lens made by plastic materials rises to change the refractive index of the objective lens, spherical aberration of the objective lens is deteriorated. Since an amount of the deterioration of the spherical aberration caused by the change of the refractive index is in proportion to the fourth power of the numerical aperture of the objective lens, the spherical aberration is deteriorated more by the change of the refractive index when an objective lens with numerical aperture 0.85 used for high density DVD is made to be a plastic lens.
Though the inventors of the invention have proposed a plastic lens with the structure of one element in one group as disclosed in TOKUGAN No. 2001-324673 and a plastic lens with the structure of two elements in two groups as disclosed in TOKKAI No. 2001-256422, as a plastic objective lens to be used for high density DVD, these objective lens each having a numerical aperture of 0.85 have a problem in practical use that a range of usable temperatures is extremely narrow for the aforementioned reason.
Explanation will be given as follows, with some examples. FIG. 1 shows how the spherical aberration component of wave-front aberration is changed by the temperature change, in the plastic lenses with the structure of one element in one group and two elements in two groups. They have focal length of 1.76 mm, and standard designed wavelength of 405 nm, and standard designed temperature 25° C. In order for information to be capable of being recorded and reproduced for an optical disk in the optical pickup device, it is necessary that the total power of the optical pickup optical system satisfies Marechal's criterion (wavefront aberration is 0.07 λ rms or less when a wavelength is represented by λ). In the actual optical pick-up optical system, there are included optical elements other than the objective lens such as a collimator and a prism, thus, the upper limit of the power (residual wave-front aberration) allowed for the objective lens is about 0.03 λ rms. From FIG. 1, in the case of the objective lens with the structure of one element in one group, 0.03 λ rms is exceeded by the change of temperature of about +10° C. On the other hand, in the case of the objective lens with the structure of two elements in two groups, its deterioration is not so remarkable as in the objective lens with the structure of one element in one group, 0.03 λ rms is exceeded for 85° C. that is the temperature that secures operations of the optical disk player. Therefore, for using a plastic lens with a high numerical aperture as an objective lens for optical pickup use that is carried in an optical disk player for high density DVD, some devices are needed.
The inventors of the invention have proposed an optical pickup device for a high density optical disk which is equipped with a light source and an objective lens having therein at least one plastic lens wherein a beam expander of a two-group structure representing an aberration correction element is provided between the light source and the objective lens (TOKKAI No. 2002-82280). Owing to this optical pickup device, spherical aberration that is deteriorated when the ambient temperature rises and the refractive index of the plastic lens included in the objective lens is changed can be corrected by adjusting the distance between lenses of the beam expander, which makes it possible to use a plastic lens having a high numerical aperture as an objective lens for the optical pickup device.
In the optical pickup device, however, there have been problems that manufacturing cost is increased by an increase in the number of parts of the optical pickup device and the optical pickup device needs to be large in size, because a spherical aberration detection means for detecting the change of spherical aberration of a plastic lens caused by the change in ambient temperature and an actuator for adjusting the lens distance of a beam expander based on results of the detection, are needed.
Incidentally, when a violet semiconductor laser that emits light with a short wavelength of about 400 nm is used as a light source, chromatic aberration caused on an objective lens brings about a problem. In the optical pickup device, a laser beam emitted from a semiconductor laser is generally of a single wavelength (single mode), and therefore, chromatic aberration on the objective lens is considered not to be a problem. Actually, however, temperature changes or output changes cause mode hopping wherein a central wavelength changes instantaneously by several nm. Since the mode hopping is an instantaneous wavelength change that cannot be followed by the focusing mechanism of the objective lens, if chromatic aberration of the objective lens is not corrected, defocus component corresponding to an amount of movement of best focus position is added, and light-converging power of the objective lens is deteriorated.
Since dispersion of general lens materials used for an objective lens is not so great in 600 nm to 800 nm representing a wavelength area of an infrared semiconductor laser or a red semiconductor laser, deterioration of light-converging power of an objective lens caused by mode hopping was not a problem.
However, in the vicinity of 400 nm that is a wavelength area of a violet semiconductor laser, dispersion of lens materials becomes extremely great. Therefore, even a wavelength change of only several nm can cause best focus position of an objective lens to be shifted greatly. In high density DVD, therefore, when mode hopping is caused on a semiconductor laser light source, light-converging power of an objective lens is deteriorated greatly, resulting in a fear that recording and reproducing cannot be conducted stably.
FIG. 2 is a diagram showing changes of wave-front aberration including defocus component of an objective lens on the occasion where a wavelength of light entering the objective lens is changed from standard designed wavelength (405 nm) with respect to the aforesaid two plastic lenses. Though it is considered that a wavelength of a violet semiconductor laser is changed by mode hopping by about 1 nm, FIG. 2 shows that the wave-front aberration including defocus component in the case of a change of a wavelength by 1 nm exceeds 0.03 λ rms for the two lenses. In the optical pickup device for a high density optical disk, therefore, even in the case that mode hopping is caused on a violet semiconductor laser light source, light-converging power is less deteriorated and recording and reproducing can be conducted stably, thus, it can be said that chromatic aberration of the objective lens needs to be corrected.